Critical Move #6

This move is from today's game in the last round of Sinquefield Cup. Carlsen only needed a draw today against Aronian to win the first Sinquefield Cup, while Aronian had to win. The interesting move was after Aronian played 30. ... Rd4 offering an exchange and Carlsen responded with a brilliant move, can you guess this move?

The puzzle is not what was actually played in the game. After done with the puzzle, click on the Move List to see the game continuation.